Silicon Carbide
The superior properties of silicon carbide, as compared with silicon, make it a perspective material for high power and high-temperature electronics (e.g. high-power transistors, thyristors, devices with P-type and N-type conductivity layers (e.g. diodes), and rectifiers). Due to an extremely high thermal conductivity (3.9 W/cm*K for SiC, vs. 1.3 for Si) and high breakdown voltage (1 MV/cm for SiC, vs. 0.3 MV/cm for Si), the SiC-based device structures are able to operate at much higher terminal voltages and output powers. The wide bandgap of SiC (>3.0 eV for hexagonal SiC, vs. 1.1 eV for Si) provides a low leakage current of the p-n junction, even at high temperatures. In addition, SiC exhibits a remarkable mechanical and chemical stability.
Despite the obvious advantages, wide-scale application of SiC in the device industry is currently hindered by difficulties arising in manufacturing of SiC-based structures of the required high quality (and by their high costs). The improvement of the quality of the growing epitaxial layers seems to be the most important task at the moment. This task includes the achievement of a good surface morphology, high thickness uniformity, an accurate stoichiometry, and a low defect density of the epilayers.
One item that presently hinders the realization of stable bipolar devices (for example) is stacking faults, which are generated from basal plane dislocations that have propagated from the substrate into the active region of the device during epitaxial growth. While it is difficult to suppress the nucleation of stacking faults, if the epitaxy is performed on on-axis substrates the basal plane dislocations terminating in the active region will be substantially reduced due to geometrical considerations. Considering on-axis substrates, the basal plane dislocations are more efficiently converted into relatively harmless threading edge dislocations, as opposed to off-axis substrate growth where many basal plane dislocations remain in the active structure and are subsequently converted into stacking faults during device operation. In on-axis growth, basal plane defects are effectively converted, resulting in improved device performance and reliability.
Defects in SiC
Commercial quality SiC wafers and epilayers include threading screw, threading edge, and basal plane (which can have edge and screw components) dislocations. Threading dislocations propagate with a component parallel to the c-axis, while dislocations that lie perpendicular to the c-plane are termed basal-plane dislocations. In SiC, it is energetically favorable for the basal plane dislocations to decompose into partial dislocations which bound a planar stacking fault defect. These stacking faults, if present in the active region of the device, result in devices with functional properties that can change unpredictably during operation. However, if the basal-plane dislocations are efficiently converted into threading edge dislocations, the “killer” stacking faults will not be generated. Efficient conversion of basal plane dislocation to threading edge dislocation can occur if on-axis epitaxy can be accomplished.
Polytypes of SiC
Silicon carbide can form into over 150 different polytypes. The main forms are 4H, 6H, and 3C (the cubic form). In the absence of growth steps, the 3C polytype forms during epitaxy. Growth steps are produced by screw dislocations, substrate miscut/cut, or preferential etching.
On-Axis Epitaxial Growth of 4H—SiC and 6H—SiC
High temperature epitaxial growth of 4H—SiC and 6H—SiC is normally performed using wafers which have been miscut at an angle of 8 or 4 degrees, respectively, toward the (1100) or (1,1,2b,0) direction. The miscut substrates are used in order to produce a high density of growth steps which are available for atomic attachment at kink sites located on the steps. It was found that if “on-axis” wafers are used, the growth is three dimensional due to high supersaturation of the growth layer. As a result a high density of 3C polytype inclusions or a poor morphology due to 2D nucleation will be produced.